1. Field of the Invention
The present invention relates to an electronic endoscopic apparatus that includes an endoscope on which an imaging unit is mounted and an image processor performing predetermined image processing on a video signal from the endoscope.
Priority is claimed on Japanese Patent Application No. 2011-136411 filed on Jun. 20, 2011, the contents of which are incorporated herein by reference.
2. Description of Related Art
In recent years, the number of pixels of solid-state imaging devices, such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor) sensor, has increased with the advance of semiconductor technology. This tendency is not an exception even in electronic endoscope apparatuses, and the number of pixels of solid-state imaging devices included in the electronic endoscope apparatuses is increasing.
With an increase in the number of pixels of the solid-state imaging devices, the frequency of a clock signal that is required for image processing has also become higher, and causing various problems as a result. For example, in the structure of an electronic endoscope apparatus, the distal end portion of a scope on which a solid-state imaging device is mounted, and an image processor that performs image processing are separated, and signal degradation on a transmission path between the solid-state imaging device and the image processor has a tendency to occur. Additionally, if the frequency of the signal becomes high, the influence of signal degradation on the transmission path between the solid-state imaging device and the image processor becomes greater still. Additionally, leakage of electromagnetic waves caused by high frequency signals flowing through the transmission path between the solid-state imaging device and the image processor also becomes more significant.
An electronic endoscopic apparatus is proposed in Japanese Unexamined Patent Application Publication No. 2001-275956. In this electronic endoscopic apparatus, a waveform smoothing circuit is inserted into an output portion of an electronic scope. Due to this waveform smoothing circuit, high-frequency noise released between the electronic scope and a processor device is inhibited.
Japanese Unexamined Patent Application Publication No. 2001-275956 contains no teaching in terms of synchronization between an endoscope and a monitor instrument. Since a solid-state imaging device having various angles of view depending on a target to be observed and use is mounted on the endoscope, an operating frequency and an angle of view are different according to the endoscope. Accordingly, to display an image captured by the endoscope on the monitor, frequency conversion adapted to a synchronization signal of the monitor is required.
Depending on a relation between a display clock and an imaging clock, there is a subtle difference between a cycle in which the endoscope captures an image of one frame and a cycle in which the monitor instrument displays an image of one frame. As such, the two cycles are gradually shifted in phase. Further, when the phase shift between the two cycles exceeds a time of one frame, a phenomenon called “passing” or “frame dropping” takes place.
FIG. 10 schematically shows a relation between a one-frame cycle based on an imaging clock and a one-frame cycle based on a display clock. As shown in FIG. 10, the one-frame cycle based on the imaging clock and the one-frame cycle based on the display clock are subtly different from each other, and a shift of the one-frame cycle (D0, D1, D2) increases with the lapse of time.